Definition 4 (IND-CPA [1]}) 
Let \(\Pi \!=\!(\mathsf {KeyGen},\mathsf {Enc},\mathsf {Dec})\)  be public-key encryption.
Define a game between an attacker \(\mathcal {A}=(\mathcal {A}_{1},\mathcal {A}_{2})\)  and a challenger as follows.
